feeder_2 enhancement

[oricdev]

Requirements: Issue #1 implemented

feeder_2 should be able to work in 2 different stages:

1. init stage: when creating a new Prosim-db: in the same way as feeder_1, extracts all products from OFF official db (which can be compared one-to-another with non-empty criteria, see issue rewrite feeders in Java 8 #1)
2. incremental stage: based on the date of last extraction, extracts ONLY all new and updated products from the OFF-official db since that date

Saves all these data into updated_products.json.

In addition, saves somewhere the date and time of last extraction and import into Prosim so that (2) can work on a subset of products instead the next time it runs.

[oricdev]

question to OFF team: is there an easy way to get in Mongo all product codes which are new or whose content was updated from a specific date and time?